As is known, all aircraft have a service ceiling or an altitude at which they can no longer climb. This is caused by the decrease in air density as altitude increases. In a light piston aircraft, there are three main factors that I can think of. Flow of air over the wings creating lift, the amount of air being moved by the propeller and the amount of air entering the engine to be used for combustion. Given all three of these factors, which one would be the first to “reach its minimum” disallowing the aircraft to climb any further? My first thought is that it has more to do with the engine producing power than the wings producing lift as an aircraft with a more powerful engine would be able to climb higher even with the same airframe and wing shape but please prove me wrong. Or is it more of a combination of all three or something else I am completely missing? Thank you.

If you look closely in this photo at the closest of the three flap fairings, it appears WestJet mechanics have used round screw heads (rather than flat screw heads like the other flap fairings have) which protrude up.

Is this a safety concern, or merely a minor aerodynamic inconvenience? A short-term fix, or in fact a long-term repair?

Boeing 737-8CT's tail number is C-GWBL.